1. Field of the Invention
The present invention relates to a technique for adjusting the control parameters of the feedback control system provided in a magnetic suspension and balance system automatically to an optimum value within a given stable searching range.
2. Description of the Related Art
A wind tunnel magnetic suspension and balance system is a device for supporting a wind tunnel model in a desired position by magnetic force, without requiring supporting structure such as support struts or stings. Such magnetic suspension and balance systems have attracted attention since they can provide an ideal environment for finding the drag coefficient of an object or measuring the conditions of wake, since there is no interference between the airflow and the supporting structure. Furthermore, they are expected to present opportunities for new developments in that they make it possible to apply desired movement to a model in a wind tunnel. Such magnetic suspension and balance systems are provided with a control system for supporting a model in a desired position by responding to any changes that may occur in conditions in the wind tunnel. The control parameters of the feedback control system that drives this magnetic suspension system are important factors in determining the stability of the supported model. These control parameters can be provisionally calculated by the equations of motion of the magnetic suspension and balance system and empirical rules, but, since the equations of motion contain uncertainties such as errors of parameters, the calculated values are rough estimations, and must be adjusted. At the present time there is no alternative to a method in which the adjustment of the control parameters is performed manually whilst observing the response of the model on site; however, such adjustment requires a considerable amount of time and, in addition, there is no guarantee that an optimum value will be obtained.
On the other hand, regarding automatic adjustment of the control parameters, a method called iterative feedback tuning has been developed in recent years and studies have been carried out aimed at applying this to various systems. However, there are no examples of application of this technique to a magnetic suspension and balance system and only a handful of studies have therefore been made regarding maintenance of stability. Also, in the case of a magnetic suspension and balance system, the problem was experienced that optimum control parameters could not in fact be obtained when the iterative feedback tuning method was applied without modification to the case of four control parameters.
In Non-patent Reference 1 (“PID control” by Nobuhide Suda, compiled by the Association for System Control Information of Japan, Asakura Shoten, 1993), a comparative table (Table 5.1 on p. 108) is presented of systems of automatic adjustment of PID control as adopted by Toshiba, Hitachi, Fuji Denki, Mitsubishi Heavy Industries, Yokogawa and Yamatake Honeywell. Identification of subjects of control (i.e. finding a mathematical model capable of representing the input/output relationships) has been performed by Mitsubishi Heavy Industries and others, but increased complexity of the program and an increased amount of calculation may be anticipated. Also, Mitsubishi Heavy Industries adopt the technique of setting a single performance index in respect of a single control parameter, so, if a plurality of controllers are required, further performance indexes must be set up, to the number of the controllers. This therefore results in the problem of a system of large overall size.
An example of a method for guaranteeing stability of an iterative feedback tuning system is reported in Non-patent Reference 2 (Iterative Feedback Tuning with Guaranteed Stability, Franky De Bruyne and Leonardo C. Kammer, Proceedings of the American Control Conference, pp. 3317-3321, 1999). This is a method of evaluating the margin of stability of a system (i.e. the gain margin and phase margin) simultaneously with performance of ordinary iterative feedback tuning. However, there may be a problem regarding increase of the amount of calculation required to calculate the stability margin in this case.